We demonstrate a new technique for absolute distance measurement with a femtosecond frequency comb laser, based on unraveling the output of an interferometer to distinct comb modes with 1 GHz spacing. From the fringe patterns that are captured with a camera, a distance is derived by combining spectral and homodyne interferometry, exploiting about 9000 continuous wave lasers. This results in a measurement accuracy far within an optical fringe (λ/30), combined with a large range of nonambiguity (15 cm). Our technique merges multiwavelength interferometry and spectral interferometry, within a single scheme.

We experimentally demonstrate that a femtosecond frequency comb laser can be applied as a tool for longdistance measurement in air. Our method is based on the measurement of cross correlation between individual pulses in a Michelson interferometer. From the position of the correlation functions, distances of up to 50 m have been measured. We have compared this measurement to a counting laser interferometer, showing an agreement with the measured distance within 2 m (4 ϫ 10 −8 at 50 m). © 2009 Optical Society of America OCIS codes: 320.7100, 320.2250, 320.1590 Traditional techniques for long-distance measurements are often based on optical interferometry when the demands on accuracy rise. Most of these interferometric techniques rely on incremental measurements of phase accumulation. A priori knowledge of the distance to be measured is required or a complex multiwavelength system may be needed. In 2004, Ye [1] proposed a simple scheme for measuring long distances in space with a stabilized femtosecond frequency comb. The scheme is based on a Michelsontype interferometry with optical interference between individual pulses. The technique proposed by Ye has been demonstrated for interferometric measurement of short displacement [2,3]. The main advantage of applying a frequency comb for distance measurement is the large range of nonambiguity, which is determined by the cavity length of the pulsed laser, ranging from about 30 cm to 3 m. It is thus not necessary to rely on incremental measurement of the optical phase. The ambiguity is easily overcome by, e.g., a laser distance meter. The stabilized frequency comb has been applied as a source in various distance measurement schemes [4,5]. In this Letter, we demonstrate distance measurements of up to 50 m in air by analyzing the cross correlation between pulses emitted from a stabilized frequency comb source. We have implemented a model of pulse propagation in air to account for the effect of air dispersion on the measured cross-correlation functions. The measurement results obtained with the frequency comb and a conventional counting laser interferometer are compared.A mode-locked Ti:sapphire laser is the frequency comb source, with both the repetition frequency and the carrier-envelop offset (CEO) frequency referenced to a cesium atomic clock (Fig. 1). The pulse duration is 40 fs, and the repetition rate f r is locked at approximately 1 GHz, corresponding to a pulse to pulse distance l pp = c / ͑n g f r ͒ of 30 cm. Here c is the speed of light in vacuum and n g is the group refractive index at the center wavelength. The CEO frequency f 0 is fixed at 180 MHz. The center wavelength of the pulses is 815 nm, with an FWHM of about 20 nm. After collimation the beam is sent to a Michelson interferometer. One part of the beam is reflected by a hollow corner cube mounted on a piezoelectric transducer (PZT) along the short reference arm. The length of the short arm can be scanned by a translation stage. The other part of the beam is reflected by two mirrors and propagates along...

We experimentally demonstrate long distance measurements with a femtosecond frequency comb laser using dispersive interferometry. The distance is derived from the unwrapped spectral phase of the dispersed interferometer output and the repetition frequency of the laser. For an interferometer length of 50 m this approach has been compared to an independent phase counting laser interferometer. The obtained mutual agreement is better than 1.5 μm (3 × 10 −8 ), with a statistical averaging of less than 200 nm. Our experiments demonstrate that dispersive interferometry with a frequency comb laser is a powerful method for accurate and non-incremental measurement of long distances.

Abstract:Finding a path towards a more accurate prediction of light propagation in human skin remains an aspiration of biomedical scientists working on cutaneous applications both for diagnostic and therapeutic reasons. The objective of this study was to investigate variability of the optical properties of human skin compartments reported in literature, to explore the underlying rational of this variability and to propose a dataset of values, to better represent an in vivo case and recommend a solution towards a more accurate prediction of light propagation through cutaneous compartments. To achieve this, we undertook a novel, logical yet simple approach. We first reviewed scientific articles published between 1981 and 2013 that reported on skin optical properties, to reveal the spread in the reported quantitative values. We found variations of up to 100-fold. Then we extracted the most trust-worthy datasets guided by a rule that the spectral properties should reflect the specific biochemical composition of each of the skin layers. This resulted in the narrowing of the spread in the calculated photon densities to 6-fold. We conclude with a recommendation to use the identified most robust datasets when estimating light propagation in human skin using Monte Carlo simulations. Alternatively, otherwise follow our proposed strategy to screen any new datasets to determine their biological relevance.